1. Field of the Invention
The present invention relates to magnetic separation processes and apparatus and particularly to magnetic separation at elevated temperatures.
2. Related Art
A key shortcoming of traditional magnetic drum separator designs is that the magnets themselves can't survive the elevated temperatures caused by hot material being fed onto the drum, thus the process requires cooling the materials before they can be magnetically separated. The normal limit for many rare earth magnetic drum separators is an operating temperature of 120 degrees Celsius. This area of the industry would need to operate a magnetic separator with feed temperatures in the range of 700 degree Celsius.
There are several temperature points that must be considered in the design of a permanent magnet assembly for a hot magnetic separator:    1. The ambient operating temperature of the magnets is important in material selection.    2. The maximum operating temperature is the temperature at which the magnets can operate more or less indefinitely without degradation of the strength of the magnetic field.    3. For any particular magnet there is a known temperature that results in the onset of degradation and/or demagnetization to a paramagnetic material.    4. The Curie point is the temperature at which the magnets become fully demagnetized into a paramagnetic material.    5. The magnets accordingly should be maintained substantially below their Curie point of the permanent magnets, i.e., equal to or less than 50% of the Curie point temperature.
Magnetic drum separators are well known in the industry. The low intensity dry versions are used to sort highly magnetic material from a material stream, often used to protect the material stream from “tramp iron”. Tramp iron, for instance, may be bits and pieces of machinery, or dropped nuts and bolts that should be removed for safety or quality reasons from a material stream. Other higher intensity magnetic drum separators are used to concentrate various magnetic minerals, such as iron ore, and separate some less magnetic materials, such as Ilmenite and garnet (magnetic product) from silica and other contaminants (non-magnetic product). See U.S. Pat. No. 6,062,393.
One physical property of a mineral is its degree of magnetic susceptibility, i.e. the general reference to minerals as being magnetic or non-magnetic. Within the mineral separation arena, materials are further defined based on their varying degrees of magnetic susceptibility. Minerals with varying degrees of magnetic susceptibility can be selectively separated with different stages and types of magnetic separation. The lower strength magnets are employed early in the process to remove the highly magnetic fractions. The following stage or stages utilize greater magnetic fields to capture the less magnetically susceptible minerals.
Some minerals are processed and transformed into other products at high temperatures using special techniques and chemistry. These processes produce product streams that become feedstock for downstream processes, like with pigments or iron and steel manufacturing. The materials separated in these instances, for example, are highly magnetically susceptible iron and highly magnetically susceptible, partially metalized, ilmenite from char, silica, and other contaminants. The industry currently makes this separation at low and reasonable temperatures, but the process economics would benefit greatly if this mixture did not require cooling prior to separation, and subsequent re-heating before being re-introduced into thermal reactors. If one could make this separation at an elevated temperature, one would save all thermal energy lost in the cooling process and all of the re-heat energy. Further savings would come from reduced capital costs for the cooling and reheating equipment.
Currently, there is a technology gap within the minerals processing industry. There is a significant need to magnetically separate materials at as high a temperature as possible. The upper limit for the temperature of this magnetic process is the Curie point or Curie temperature of the magnetic components of the mixture, which is the point where certain magnetic materials undergo a sharp change in the magnetic properties of the material.
The Curie temperature for pure iron is known to be 1043K or about 770 degrees Celsius. For this reason, it was determined the hot magnetic separation process herein needs to manage feed temperatures of up to about 700 degrees Celsius.
The conventional method for manufacturing a magnetic drum separator is as follows: A manufacturer creates a cylindrical drum that rotates on its longitudinal axis utilizing end plates and bearings. This drum assembly rotates on a stationary shaft that also supports the magnetic assembly inside the drum. This way, the drum rotates over a stationary magnet housed inside the rotating drum. The clearance between the inside of the drum and the surface of the magnet is usually minimized to maximize the magnetic field outside the drum, maximizing the separation effect. See U.S. Pat. No. 6,062,393. It is important that this shell be as thin as practical, non-magnetic and wear resistant. The most common material for the shell of this drum assembly is thin stainless steel with a typical thickness of about 3 mm. The most common material for the end plates of the drum is aluminum plate, usually about 19 mm thick. Bearings are attached to the end plates that allow the drum assembly to rotate on the horizontal stationary shaft. These bearings are commonly ball or roller bearings and are either sealed or allow for grease addition for lubrication. The stationary shaft is held in clamps that allow the operator to position the magnetic section for best effectiveness. The magnetic section usually has a pie shape when viewed from its end, and the radius of the magnetic section closely matches the inside radius of the shell. Many separations require maximum magnetic effect so the magnet to shell clearance is minimized. The magnetic section is commonly made from a combination of high strength permanent magnet blocks arranged to maximize the magnetic performance outside the shell.
During operation, the material mixture to be separated is fed in a continuous stream, in the form of a granular or lumpy mixture, directly onto the drum surface, as the drum rotates on its horizontal axis. The drum is rotated using a drive system commonly consisting of a motor and a gearbox sometimes aided by drive belts and pulleys. The feed is normally presented to the rotating drum surface at the twelve o'clock position using a vibratory or rotary feeder and a feed chute. The feed is presented to the drum in a direction that is approximately tangent to the drum surface, and in the direction of rotation. It is desirable to closely match the velocity of the feed material to the velocity of the drum surface to minimize both wear of the drum surface and skipping or bouncing of the particles. Minimizing skipping and bouncing of feed particles improves the separation performance of the magnetic drum separator and reduces wear. Rotation of the drum commonly ranges between 20 and 70 revolutions per minute (rpm) for a drum diameter of about 610 mm.
As the material travels on the drum, the magnetic particles are attracted to the magnet and tend to stick to the drum. The non-magnetic particles tend to leave the surface of the drum through centrifugal forces. Feed materials then take different trajectories based on the degree of magnetic susceptibility, and other physical properties such as mass, shape and density. The operator then selects the positions of one or more movable splitters that direct the material to different hoppers. The most common arrangements are to have either one or two splitters that divide the material into either two products of magnetic and non-magnetic, or three products called magnetic, non-magnetic and middlings. These products are directed away for delivery to a customer, for further processing, or to the scrap or tailings pile.
Another approach of the prior art is illustrated in U.S. Pat. No. 4,000,060 to Collin. This patent seems to suggest the passage of steam and perhaps contaminates from inside the drum and out through the bearings. The bearings would have to be designed to withstand such harsh service. While it is true that such an approach would appear to prevent particulates and hot gases from entering the bearings, the present design employs clean fresh gaseous nitrogen from inside the drum to escape out and through the bearings to cool the bearings and to prevent egress of particulates from entering the bearings from external sources. Therefore the approach of the present invention is clearly preferred.
In addition, Collin '060 includes spraying water within the drum and creating steam to produce a cooling method. The difficulty with that approach involves the fact that rare earth magnets corrode or rust readily in an environment that includes moisture and water.
In the present invention liquid cooling is placed inside a cooling tube circuit instead of allowing direct contact with the magnets. Furthermore, the use of boiling water raises a large number of issues regarding water quality and chemistry control. In addition to scale buildup, there are issues regarding safety and control of boiling water systems. Finally, the accumulation of solids can interfere with close tolerances that exist in the system.
A number of disadvantages arise in the material pickup structure and method of the Collin patent '060, namely:    1. First, trying to lift the magnetic products against gravity and with the assistance of the gas flow exiting at 3 is difficult. Falling material schemes and splitters to divide factions are preferred because of the better control of all three factions when you optimize the separation by manipulating rpm, feed rate, temperature, and splitter positions.    2. Collin '060 needs to pressurize the hot box or cabinet to provide for the escape of gas at 3. The pressure and flow rate that is optimum for the material removal at 3 might not be optimum for the fluidization at 4a. Also perfect fluidizing is required to get all particles near the magnetic field so that sorting can occur. This can be accomplished, but as the particle size increases, it becomes more difficult. The small particles will easily fluidize and the big particles will not. The design of this invention rejects this prior art approach and uses the dry drum approach as well known in the prior art, by requiring all particles to pass through the magnetic zone without regard to particle size.    3. An additional issue arises when one is boiling water inside a rotating shell. The temperature of the water and the steam above the water must necessarily be about 100 degrees C. This is near ideal for the better rare earth magnets as they often have a maximum operating temperature of 120 degrees C. The instant design is much better for two reasons: during testing with a significant heat load on the instant shell, the instant magnet assembly remained very near to the coolant temperature of about 10 degrees C. Thus a great deal of safety margin is provided compared to that 120 degrees C. maximum operating temperature of Collin '060. Also, the cooled shell of Collin will very significantly cool the magnetic products and the nonmagnetic products labeled M in FIG. 1, because the shell will be at a temperature of about 100 degrees C. The instant design will accomplish much less cooling, because the shell is at or very near the feed temperature of about 700 degrees C., resulting in low heat transfer out of the feed particles, while maintaining the magnets in a cool state to survive for a long life.